esbuild

Bun's bundler API is heavily inspired by esbuild. This page is a side-by-side comparison of the two APIs.

A few behaviors differ:

Performance

Bun's bundler is 1.75x faster than esbuild on esbuild's three.js benchmark.

CLI API

# esbuild
esbuild <entrypoint> --outdir=out --bundle

# bun
bun build <entrypoint> --outdir=out

In Bun's CLI, boolean flags like --minify take no argument. Flags that take one, like --outdir <path>, can be written as --outdir out or --outdir=out. Some flags, like --define, can be repeated: --define foo=bar --define bar=baz.

JavaScript API

Plugin API

Bun's plugin API is designed to be esbuild-compatible. Bun doesn't support esbuild's entire plugin API surface, but it implements the core functionality. Many third-party esbuild plugins work with Bun without modification.

In both Bun and esbuild, you define plugins with a builder object.

myPlugin.tsCopy

import type { BunPlugin } from "bun";

const myPlugin: BunPlugin = {
  name: "my-plugin",
  setup(builder) {
    // define plugin
  },
};

The builder object's methods hook into parts of the bundling process. Bun implements onStart, onEnd, onResolve, and onLoad; it does not implement the esbuild hooks onDispose and resolve. Bun partially implements initialOptions: the object is read-only and exposes only a subset of esbuild's options. Use config (the same thing in Bun's BuildConfig format) instead.

myPlugin.tsCopy

import type { BunPlugin } from "bun";
const myPlugin: BunPlugin = {
  name: "my-plugin",
  setup(builder) {
    builder.onStart(() => {
      /* called when the bundle starts */
    });
    builder.onResolve({/* onResolve.options */}, args => {
      return {/* onResolve.results */};
    });
    builder.onLoad({/* onLoad.options */}, args => {
      return {/* onLoad.results */};
    });
    builder.onEnd(result => {
      /* called when the bundle is complete */
    });
  },
};

onResolve

onLoad